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chapter 4 Photosynthesis

chapter 4 Photosynthesis. Introduction Strategies for getting energy Photoautotrophic & Chemoautotrophic: • Make it (e.g. photosynthesis) Heterotrophic. • Eat it. CO ２ + H ２ O 光 绿色植物 (CH ２ O) ＋ O ２ (1). Introduction

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chapter 4 Photosynthesis

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  1. chapter 4Photosynthesis

  2. Introduction • Strategies for getting energy • Photoautotrophic & Chemoautotrophic: • • Make it (e.g. photosynthesis) • Heterotrophic. • • Eat it CO２+ H２O 光 绿色植物(CH２O)＋O２ (1)

  3. Introduction • Sunlight = energy & information for plants. • Radiant energy (SUNLIGHT) • • Maintains planetary temperature suitable for life • • Ultimate source of energy to sustain life (Photosynthesis). • Provides information about the environment • • Photomorphogenisis (光形态发生作用） – light quality. • • Photoperiodism-- day/night length.

  4. Significance of photosynthesis • Photosynthesis – Essential for animal survival. • • Captures energy from the sun. • • Makes complex organic molecules from CO2, water and minerals. • • Releases O2 required for cellular respiration. • ~ 250 Billion metric tons of sugar produced each year via photosynthesis

  5. Photosynthesis light 6 CO2 + 6 H2O C6H12O6 + 6 O2 Consists of both Light & Carbon-fixation Reactions

  6. Light – Properties • Visible light comprises a small portion of the electromagnetic spectrum (光谱）.

  7. Light Properties • Ultraviolet (UV) light => 100 - 400 nm • Photosynthetically Active Radiation • (PAR) => 400 -- 700 nm • • Violet => ~380 -- 455 nm • • Blue => ~455– 500 nm • • Green => ~500– 580 nm • • Yellow => ~580– 595 nm • • Orange => ~595– 620 nm • • Red => ~620– 700 nm • • Far Red => ~700– 775 nm • Infrared => ~775– 100,000 nm

  8. Light Properties • The particle aspect of light is described as a photon（光子）. • • Each photon contains specific amount of energy called a quantum (量子) (plural quanta). • • The energy of a photon (a quantum of light) is inversely proportional to its wavelength (λ). • • Violet light (short λ) has almost twice the • energy of red light (long λ).

  9. 3 Ways to Describe Light • Light quantity-- how much? • Light quality --composition with respect to wavelength. • Timing - duration and periodicity.

  10. Chloroplasts Review • One of a family of organelles (the plastids). • Bound by a double membrane. • Contains chlorophyll, DNA.

  11. Chloroplast Structure • Double Membrane • • Outer - highly permeable. • • Inner – selectively permeable.

  12. Chloroplast Structure • Stroma - background matrix. • • Contains enzymes for capturing (fixing) CO2 (e.g. RUBISCO) during carbon fixation. • • Contains DNA, RNA, ribosomes and machinery for transcription & translation （翻译）. • Thylakoids - Internal membrane system. • • Grana - membrane stacks. • • Stroma lamella - provide network connections between grana. • • Where the light reactions occur.

  13. Chloroplast Structure • Lumen （腔）– intra-thylakoid space. • Site of H2O oxidation - source of evolved photosynthetic oxygen. • Reservoir for protons pumped across thylakoids. • • Electron transport, ATP synthesis.

  14. Pigments • Pigments = light absorbing substances. • All pigments active in photosynthesis are found in the chloroplasts.

  15. Chlorophyll • Embedded in the thylakoid membranes. • Absorbs light strongest in the violet/blue and red wavelengths. Reflect green light. • Chlorophyll a– occurs in all photosynthetic eukaryotes and cyanobacteria. • Main pigment responsible for transforming light into chemical energy.

  16. Accessory Pigments • Pigments not directly involved in photosynthetic energy transduction in the reaction center. • They capture light energy of different wavelengths and then pass the energy to chlorophyll a in the reaction center. • Chlorophyll b - plants, green algae, and euglenoid algae. • Chlorophyll c - some groups of algae.

  17. Accessory Pigments • Carotenoids (类胡萝卜素)-- • Embedded in the thylakoid membranes. • Also serve a critical function in preventing photooxidative damage to chlorophyll (anti- oxidant). • Two groups • • Carotenes – Beta -carotene is a key source of vitamin A. • • Xanthophylls – Anti-oxidants. Accessory Pigments • Phycobillins (billinpigments)– • cyanobacteria & red algae. • • Water soluble

  18. Another Pigment • Anthocyanins (花青素)- flower petal color. • • Stored in the vacuole. • • Not involved in photosynthesis. • Not an Accessory Pigment.

  19. Energy Transfer • Once light energy (quanta) is absorbed, as the e- drops back to ground state, the energy can be: • • Lost as heat. • • Lost as light (fluorescence) at slightly lower wavelength + a little heat. • • Transferred to another molecule by exciting the other molecule’s e-. • • Transferred to another molecule by passing the excited e- itself. • The last two can be used in photochemical reactions powering photosynthesis.

  20. Fluorescence

  21. Photosynthesis • = Two Steps • Light Reactions • • Requires light. • • Produces ATP and NADPH (energy stored in chemical bonds) to be used in CO2 fixation. • Carbon-Fixation Reactions • • ATP and NADPH produced in the light reactions are used to fix and reduce carbon into simple sugars.

  22. The Light Reactions • Also called the: • • thylakoid reactions. • • light-dependent reactions. • • energy-transduction reactions. • Overall Function: • • Uses light energy to make ATP and NADPH (energy stored in chemical bonds) to be used in CO2 fixation.

  23. The Light Reactions • Occur on the thylakoid membrane.

  24. The Light Reactions • Consists of two photosystems: • • Photosystem II : P680 • • Photosystem I : P700 • Each has ~ 250 to 400 pigment molecules. • • Antenna complex– funnel e- to the reaction center. • • Reaction center– protein & 2 special chlorophyll a molecules that convert light energy to chemical energy.

  25. Light reactions • Primary reaction: • Energy transfer: light to electric energy • Oxidation/reduction: • Electron transport and photophosphorylation • ATP and NADPH produced

  26. The Light Reactions • Primary reaction: Energy transfer • • primary electron donor (D) • • reaction center (P) • • primary electron acceptor (A) D·P·A D·P*·A D·P+·A-D+·P·A-

  27. Primary Reaction • Oxidation/Reduction: • • Pigment is photo-oxidized. • • Acceptor molecule is reduced. • • Photo-oxidation of chlorophyll is the primary photochemical act in photosynthesis. D·P·A D·P*·A D·P+·A-D+·P·A-

  28. Electron transport • Emerson effect: cooperating photosystems • In 1950s, Robert Emerson in Illinois U • Red light of wavelength ＞690nm is ineffective in causing photosynthesis • If light of shorter wavelengths provided simultaneously, photosynthesis was faster than the sum of the two rates with either color. Synergism or enhancement

  29. Electron transport • Z - scheme:cooperating photosystems • both photosystems (photosystem I and photosystem II ) function together to conduct the electron transport, and the electron transport chain exhibits a lateral Z shape when they are arrayed by redox potential.

  30. Electron transport chain • Photosystem II • Plastoquinone (PQ) • The Cytochrome b6/f complex • Plastocyanin (PC). • Photosystm I

  31. Electron Transport Photosystem II • Oxygen evolving complex provides e- for transport. • • Located on the lumen side of the thylakoid membrane. • • Where water is split (photolysis) releasing O2 , 4 e-, and 4 H+ for every 2 H2O. • Optimum absorption peak for chlorophyll a at 680 nm (P680).

  32. Electron Transport • Plastoquinone (PQ) • • Carries an H with the e-

  33. Electron Transport Cytochromeb6/f complex • Multiprotein complex. • Evenly distributed on the membranes. • Catalyse oxidation of PQH2 and reduction of PC. • H+ from PQ accumulate in the lumen.

  34. Electron Transport • Plastocyanin (PC) • • Peripheral protein on the lumen side • • Shuttles e- between • chtochrome b6/f • complex and PS I.

  35. Photosystem I • • First photosystem discovered. • • Optimum absorption peak for chlorophyll a is 700 nm (P700). • • Primarily on stroma lamellae (non-stacked).

  36. NADP Production • PS I passes e- to Ferredoxin • • Small, water soluble Fe-S protein. • • Located on the stroma side of thylakoid. • Then to a NADP reductase. • • Forms NADPH from NADP+ and H+ • on the stroma side of thylakoid.

  37. ATP Synthesis • ATP synthase couples the transport of H+ with ATP synthesis (photophosphorylation). • H+ gradient created through: • • Photolysis of water in lumen. • • Electron transport chain (H+ into lumen). • • Use of H+ to make NADPH in stroma. • Thylakoids - quite impermeable to H+ except through ATP synthase.

  38. ATP Synthesis • As H+ flow down their concentration and electrochemical gradients, ATP synthase uses the energy to create ATP = the chemiosmotic mechanism. • ATP formed on the stroma side of the membrane

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